1. Field of the Invention
The present invention relates to an electric double layer capacitor having a small internal resistance and having the voltage-holding property improved.
2. Discussion of Background
Heretofore, as a separator for an electric double layer capacitor, a polyethylene non-woven fabric, a polypropylene non-woven fabric, a polyester non-woven fabric, kraft paper, a Manila hemp sheet or a glass fiber sheet has been known (JP-A-1-283811, JP-A-1-304719, etc.). The role of the separator is to electrically insulate the positive electrode from the negative electrode and to facilitate movement of ions in the electrolyte solution which takes place along with charging and discharging.
In recent years, an attention has been drawn to an electric double layer capacitor for heavy-current charging and discharging. However, with a separator made of the above-mentioned paper or non-woven fabric made of organic fibers, the liquid-maintaining property and the absorptivity for the electrolyte solution are inadequate, and the ion conductivity is low so that the internal resistance tends to be large. Accordingly, when instantaneous heavy-current discharge (which is one of characteristics of the electric double layer capacitor of this type), was carried out, the voltage drop was so large that the operation was impractical. Further, the liquid-maintaining property and the absorptivity for the electrolyte solution were inadequate, and the separator was likely to dry up and thus to lead to deterioration of the performance.
On the other hand, with a separator made of a glass fiber sheet, the fiber diameter is usually small as compared with organic fibers, and such a separator is excellent in wettability to the electrolyte solution and is accordingly excellent in the liquid-maintaining property and the absorptivity. Accordingly, when the separator made of a glass fiber sheet is employed, the ion conductivity will be large, and the internal resistance of the electric double layer capacitor can be made low. Further, the glass fibers do not fuse like synthetic fibers even at a high temperature and thus have a merit that the internal short circuiting due to fusion of the separator, will not occur even when the capacitor is used at a high temperature. However, glass fibers constituting the conventional glass fiber sheet are composed mainly of glass fibers having fiber diameters of at least 10 .mu.m, and the liquid-maintaining property and the liquid absorptivity are not adequate, although they are better as compared with those of the separator made of organic fibers, and deterioration in the performance of the capacitor was still likely due to drying up of the separator.
Further, with the separator of a sheet made of thick glass fibers having fiber diameters of at least 10 .mu.m, void pores are large, and fine particles of activated carbon used for an electrode can easily pass through the separator. Accordingly, charged activated carbon particles are likely to move to a counter electrode side by electrophoresis and consequently lose electric charge, thus leading to selfdischarge.